A drip irrigation system provides application of water to specific plant or root zone locations in controlled quantities. It is thereby possible to irrigate planted areas with substantially less water than is used by general broadcast sprinkler or flooding methods. However, if the same non-pressure-compensating emitter is used at all drip locations, those emitters in the irrigation system which are subject to higher water pressure release more water. There is thus uneven water distribution.
In order to overcome this problem, pressure-compensating emitters have been developed which allow a fairly uniform water flow over a range of water pressures within the irrigation system. This allows an irrigation system to be constructed having longer drip lines and fewer distribution or sub main lines. A substantial number of these use a deformable material, such as a resilient web, which restricts a flow path or distorts water outlets in order to limit flow rates.
Examples of these devices are described in Mehoudar et al. U.S. Pat. No. 4,307,841; Smith U.S. Pat. No. 4,344,576; and Delmer U.S. Pat. No. 4,354,639. Miller discloses in U.S. Pat. No. 4,824,025 an emitter having an internal transverse channel member that closes with increasing conduit fluid pressure. When the channel is restricted it is vulnerable to clogging. A more extensive list is given in my U.S. Pat. No. 4,653,695. A disadvantage of such devices is that the restricted flow path tends to become clogged due to sediment collecting in the restricted region. My patent discloses another type of pressure-compensating emitter that avoids this restriction-clogging problem. This emitter has a cylindrical member positioned in a sleeve, with a labyrinth formed between the cylinder and sleeve. A non-fluid-restricting chamber exists at one end of the cylinder so that the cylinder can slide in and out of the sleeve, and thereby vary the length of the labyrinth. The position of the cylinder is adjusted according to the water pressure, so that a longer labyrinth exists at higher pressures.
That emitter solved the problem of clogged passageways, since the passageway cross-sectional area is constant, but is relatively expensive to produce.
Other forms of emitters have also been developed. For instance, Bentley in U.S. Pat. No. 3,998,244 shows a drip emitter with a helical flow path that is manually adjustable in length. The flow path has dams that are resilient and deform in response to fluid at an inlet pressure to increase the resistance to flow.
Christy et al., in U.S. Pat. No. 4,382,549, discloses an emitter in which a flexible plate moves between a closed position and a higher pressure restricted-flow position. When the plate is in an intermediate position, water flows relatively unrestrictedly.
Samueli discloses in U.S. Pat. No. 4,600,152 an emitter in which flow is limited by intersecting flow paths. Further resistance to flow is provided by obstructions at the flow intersections. This emitter does not permit compensation for change in fluid pressure.
In Benson et al. U.S. Pat. No. 4,613,080, disclose an emitter that has various flow paths. The emitter is manually adjusted to control the number of flow paths. Once set, there is no compensation for changes in fluid pressure.
In the invention disclosed by Sanville in U.S. Pat. No. 4,817,666, a solenoid selectively opens and closes outlets. Flap valves are then responsive to pressure drop at a closed outlet to close an associated output. This is a complex and expensive form of valve.
Prassas, in U.S. Pat. No. 4,917,535 also discloses an externally adjustable emitter. A valve is manually adjustable for bypassing a restricted flow path to allow flushing the fluid outlet.
A similar device is disclosed by G. Rolland in French Pat. No. 2,625,544. At a low fluid pressure, a flexible disk allows relatively free flow of fluid over a barrier to an outlet. As pressure increases, the disk deforms, apparently restricting flow over the barrier. Ultimately flow over the barrier is stopped, but a turbulent flow path covered by the disk remains open.
Many of these prior art emitters are built with a low-pressure, free-flow feature, that allows them to be flushed out. In large irrigation systems this can require increasing the power of the pumping system to create a great enough flow to close the free-flow feature and thereby set all the emitters into a limited flow, pressurized system. This additional power makes the irrigation system more expensive.
There thus remains a need for a dynamically pressure-compensating emitter that maintains an unrestricted, fluid-flow-limiting passageway during all operating conditions and is relatively economical to produce